Liquid crystal display, surface light source device and light control sheet

ABSTRACT

A fluorescent lamp  6  emits illumination light (primary light), which is deflected by means of a light guide plate  4  to be converted into a light flux having an enlarged cross section. A liquid crystal display panel  3  is supplied with the light flux via a prism sheet (light control sheet)  12 . The prism sheet  12  or an alternative light control sheet to be employed is capable of rotating polarization involved by the light toward a direction of light transmission axis of a polarization plate  14 . An effective light input to the liquid crystal display panel  3  is achieved by light transmission through the polarization plate  14  at a high transmission rate. The liquid crystal display panel  3  controls polarization state of the inputted light and, according to the state, causes the light to pass through another polarization plate (analyser)  15 . Emission occurs, under intensity control depending on positions, via the polarization plate (analyzer)  15 , to provide an image. Polarization rotation function for modifying the emission of the light guide plate  4  can be obtained through manufacturing of a mother material of the prism sheet  12  by means of one-axle drawing process or two-axle drawing process. Prism cuts may be formed on a back face  8  or emission face  11  of the light guide plate  4 . A polarization-rotating sheet may be alternatively interposed between a prism sheet provided with no polarization-rotating ability and the polarization plate  14.

BACKGROUND

1. Field of the Invention

The present invention relates to a surface light source device, a liquidcrystal display employing the surface light source device and a lightcontrol sheet which is advantageously applied to them. The presentinvention is applied, for example, to displays for personal computer ornavigation system.

2. Related Art

Referring to FIG. 16, shown is an outlined arrangement of a liquidcrystal display 29 which is employed, for example, in a portablepersonal computer. The liquid crystal display 29 comprises a liquidcrystal display panel 32 and a surface light source device 33 disposedon a lower side (i.e. back side) of the panel. The illustrated surfacelight source device 33 is provided with a fluorescent lamp 36, lightguide plate 34 and prism sheet (light control sheet) 35.

According to a well-known operation, the fluorescent lamp 36 emitsillumination light (primary light), which is deflected by the lightguide 34 to be converted into a flux having an enlarged cross section.The flux is supplied to the liquid crystal display panel 32 via theprism sheet (light control sheet) 35.

The liquid crystal display panel 32 comprises a polarization plate 37disposed on a light input side, another polarization plate (analyser) 40disposed on a light output side and a liquid crystal display cell 38interposed between them. Accordingly, the liquid crystal display cell 38is supplied with the output illumination light of the surface lightsource device 33 via the polarization plate 37. As known well, theliquid crystal display cell 38 controls polarization state of theinputted light depending on position according to output signals of adrive circuit (not shown).

Then, output light of the liquid crystal display cell 38 transmitsthrough the polarization plate (analyser) 40 depending on state ofpolarization. After all, light H intensity of which is controlleddepending on position is emitted through the polarization plate(analyser) 40. Some of the light H is incident to eyes 41 of an operatorof the personal computer, causing the operator to see an image.

The conventional liquid crystal display 29 is, however, subject to aproblem that has been unrecognized. That is, screen brightness oftenvaries depending on combination of the liquid crystal display panel 32and the surface light source device 33 arranged for the device. It isnoted that difference in screen brightness arises depending on neitherperformance of the surface light source device 33 itself nor that ofliquid crystal display panel 32, but depending on congeniality betweenthe device 33 and panel 22. Such phenomenon has been unknown and foundnewly.

Researches tells that the phenomenon occurs in relation to direction oftransmission axis of the polarization plate 37 disposed on an input sideof the liquid crystal display cell 38. In general, a polarization plate37 is arranged in a liquid crystal display so that the transmission axis42 is orientated as shown in either FIG. 17 a or FIG. 17 b. In FIGS. 17a and 17 b, reference symbol E represents an extending direction ofprismatic grooves 35 a of a prism sheet 35 (cf: FIG. 16), the directionbeing parallel with that of the fluorescent lamp 36.

It has been found that a great difference in screen brightness arisesbetween a case as shown in FIG. 17 a (Case 1) and another case as shownin FIG. 17 b (Case 2) under employment of the same prism sheet 35 inboth cases.

Therefore, the problem seems to solved passively by means of excludingitems (liquid crystal displays) having “bad congeniality” throughbrightness checking of every individual item, or by means of changingcombination so that “good congeniality” is realized. Such ways leads,however, to a reduced efficiency of working. Thus, a positive means tosolve the problem has been awaited.

OBJECT AND SUMMARY OF INVENTION

The present invention is proposed under the above-described background.An object of the present invention is to provide a surface light sourcedevice which is improved as to overcome the above-described problem.Another object of the present invention is to provide a liquid crystaldisplay free from the above-described problem. Still another object ofthe present invention is to provide a light control sheet which iscapable of contributing to solution of the problem in the devices.

First, the present invention is applied to a liquid crystal displaycomprising a liquid crystal display panel and a surface light sourcedevice for inputting light to the liquid crystal display panel. Theliquid crystal display panel is provided with at least a liquid crystalcell and a polarization plate arranged at an input side of the liquidcrystal cell, and the surface light source device is provided with atleast a light guide plate, a primary light source to supply illuminationlight to the light guide plate and a light control sheet interposedbetween the light guide plate and the polarization plate.

According to a feature of the present invention, the light control sheetemployed in the device is one that rotates a maximum-intensity-directionof polarization involved by light emitted from the light guide platetoward a direction of light transmission axis of the polarization plate.

The present invention is also applied to a surface light source devicefor inputting light to a liquid crystal display panel provided with atleast a liquid crystal cell and a polarization plate arranged at aninput side of the liquid crystal cell. The surface light source devicecomprises at least a light guide plate, a primary light source to supplyillumination light to the light guide plate and a light control sheetinterposed between the light guide plate and the polarization plate.

According to the feature of the present invention, the light controlsheet employed in the device is one that rotates amaximum-intensity-direction of polarization involved by light emittedfrom the light guide plate toward a direction of light transmission axisof the polarization plate.

The present invention is further applied to a light control sheetarranged for inputting light to a liquid crystal display panel providedwith at least a liquid crystal cell and a polarization plate which isarranged at an input side of the liquid crystal cell, the light controlsheet being applied to a surface light source device provided with atleast a light guide plate and a primary light source to supplyillumination light to the light guide plate.

According to the feature of the present invention, the light controlsheet has a function of rotating a maximum-intensity-direction ofpolarization of light emitted from the light guide plate toward adirection of light transmission axis of the polarization plate.

The above and the other features of the present invention will beunderstood with ease from the following detailed description withreferring to the drawings attached. It should be noted that sizes ofelements are partially exaggerated as required for the sake of easyunderstanding in the drawings.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is an exploded perspective view of a liquid crystal display inaccordance with an embodiment of the present invention;

FIG. 2 is a cross section view along line A-A in FIG. 1;

FIG. 3 is an enlarged view of a portion B in FIG. 2;

FIG. 4 is an illustration of processes for manufacturing of prismsheets;

FIG. 5 is a diagram to illustrate a method for measuring polarizationstate of emission from a light guide plate;

FIG. 6 is a first result obtained in a measurement according the methodillustrated in FIG. 5;

FIG. 7 is a diagram to illustrate a method for measuring polarizationstate of light after transmitting through a prism sheet;

FIG. 8 is a diagram to illustrate cutting states of prism sheets;

FIG. 9 is a table to show screen brightness obtained in measurementswhere the prism sheets illustrated in FIG. 8 were arranged by turns in aliquid crystal display;

FIG. 10 is a graphic illustration to show polarization states ofemissions after transmitting through prism sheets 12 a to 12 c shown inFIG. 8, respectively;

FIG. 11 is a graphic illustration to show polarization states ofemissions after transmitting through prism sheets 12 f to 12 h shown inFIG. 8, respectively;

FIG. 12 is a graphic illustration to show a polarization state ofemission after transmitting through an acrylic-resin prism sheetmanufactured by means of compression molding;

FIG. 13 a is a diagram to illustrate orientations of the prism sheets 12a to 12 c with respect to a polarization plate, respectively;

FIG. 13 b is a diagram to illustrate orientations of the prism sheets 12f to 12 h with respect to a polarization plate, respectively;

FIG. 14 is a graphic illustration to compare polarization states ofemissions with each other, one of the emissions being outputted from alight guide plate having a back face provided with prismatic grooves,another being outputted from a light guide plate having an emission faceprovided with prismatic grooves, and the other being outputted from alight guide plate having back and emission faces provided with noprismatic groove;

FIG. 15 is a cross section view of a liquid crystal display whichemploys a sheet member as a sole light control sheet;

FIG. 16 is a diagram to illustrate a using state of a conventionalliquid crystal display;

FIG. 17 a is a diagram to illustrate a state of combination of apolarization plate and a prism sheet in a prior device; and,conventional liquid crystal display;

FIG. 17 b is a diagram to illustrate another state of combination of apolarization plate and a prism sheet in a prior device.

EMBODIMENTS (1) First Embodiment

FIGS. 1 and 2 show a liquid crystal display in accordance with anembodiment of the present invention. The liquid crystal display 1comprises a liquid crystal display panel 3 and a surface light sourcedevice 2 arranged for illuminating the panel. And the surface lightsource device 2 is provided with a light guide plate 4, rod-likefluorescent lamp 6, roughly-U-shaped lamp reflector 7, reflection sheet10 and prism sheet 12.

The light guide plate 4 is rectangular and has a size roughly the sameas that of the liquid crystal display panel 3. The rod-like fluorescentlamp 6 and roughly-U-shaped lamp reflector 7 compose a primary lightsource. The reflection sheet 10 is disposed along a back face 8 of thelight guide plate 4. The prism sheet 12 functions as a light controlsheet which controls and modifies direction of emission from an emissionface 11 of the light guide plate 4.

The liquid crystal display panel 3 comprises a liquid crystal cell 13, apolarization plate 14 disposed on a lower face (light input side) 13 aof the cell and another polarization plate 15 disposed on an upper face(light output side) 13 b of the cell. As illustrated in FIG. 13 a orFIG. 13 b, the polarization plate 14 is orientated so that atransmission axis 16 is inclined 45 degrees with respect to an end edge17 shown on an upper portion in the illustration (i.e. on the side ofthe fluorescent lamp 16). It is noted that “liquid crystal display cell”is an element composed of an array substrate, CF-substrate and liquidcrystal material enclosed between them.

The light guide plate 4 is made of a light-permeable resin (such aspolymethyl methacrylate) within which light scattering material such assilicone-type resin powder is dispersed uniformly, being preferablyprovided with a wedge-like cross section. A great number of prismaticgrooves 19 are formed repeatedly on the back face 8 of the light guideplate 4 in a direction approximately vertical to a longitudinaldirection of the fluorescent lamp 6. Each groove has a cross sectionlike a triangle.

To prepare the prism sheet (light control sheet) 12, the followingmanufacturing steps were performed. The steps are typical and generallyemployed.

1. Polyethylene terephthalate (PET) 20 is extruded from an extrusionmachine 18 as shown in FIG. 4 (Step of extrusion).

2. Two-axle drawing step in which a sheet material having apredetermined width is formed by two-axle drawing of the extruded PET 20and followed by rolling up.

3. Prismatic face forming step in which a prismatic face 22 made ofultraviolet-setting resin curing resin is formed continuously on any oneface 21 a of the sheet material 21 obtained in the two-axle drawingstep.

4. Cutting step in which rectangular light control sheets are obtainedby cutting the sheet material provided with the prismatic face 22 formedthrough the prismatic face forming step.

Cutting size is determined as to approximately the same as size of theemission face 11 of the light guide plate 4. The prismatic face 22 isformed in the prismatic face forming step illustrated in FIG. 4 so thatprismatic configuration extends in a direction along the drawingdirection (arrow C) of the sheet material 21. And a great number ofprismatic grooves are formed in a predetermined pitch in awidth-direction W of the sheet material 21 and in parallel to eachother.

In the cutting step, cutting-out of each prism sheet 12 from the sheetmaterial 21 is performed so that each prism sheet 12 has a prismaticface 22 provided with a prismatic configuration extending approximatelyin parallel with an incidence face of the light guide plate 4 (See FIGS.2 and 4).

It is noted that Fx represents tension in X-axis direction and Fyrepresents tension in Y-axis direction.

As follows, description is given about mechanism how the foresaidproblem arises in the prior arts and further about a method how to solvethe problem with referring to FIGS. 5 through 12.

First, angular emission intensity distribution of the light guide platewas examined. It has been observed that a maximum-intensity-direction 23is approximately perpendicular to the incidence face 5 and is inclinedabout 70 degrees to a normal 24 with respect to the emission face 11 asto be far from the fluorescent lamp 6.

Nevertheless this is merely an example, in general, there is no greatdifference between the above-observed maximum-intensity-direction andthose observed in other light guide plates. In view of this, theabove-observed maximum-intensity-direction is “typical”.

Under such a situation, a light beam emitted in themaximum-intensity-direction 23 is recognized as a representativeemission from the light guide plate 4 and is examined for polarizationstate. Measurement was carried out in an arrangement as shown in FIG. 5.

A photo-intensity measuring device 30 is composed of a polarizationplate for measurement 26 having a light transmission axis 27 and aphotometer 28 for detecting light which transmits through thepolarization plate 26. Intensity of light was measured at many rotationpositions around the direction 23. Results are shown in a graph of FIG.6. In the graph, an angular scale extending peripherally indicatesorientational positions of the light transmission axis 27 (dual-arrow inFIG. 5).

A radial scale indicates relative intensity of light detected by thephotometer 28. The full scale is 100. It is noted that such a graphicillustration format in FIG. 6 is also applied to FIGS. 10 to 12 and 14in the same manner.

When a projection of the transmission axis 27 onto the emission face 11is orientated in accordance with a direction 25 perpendicular to theincidence face 5, it corresponds to 0 degree or 180 degrees. In theillustrated case, FIG. 5 shows a state of 0 degree.

The polarization plate for measurement 26 was rotated clockwise(direction D) from 0 degree position and stepwise by 5 degrees, whereina polarization component was measured at every rotation position.

As shown in FIG. 6, the photometer 28 gave the maximum value at 0 degreeand 180 degrees respectively while the meter 28 gave the minimum valueat 90 degrees and 270 degrees respectively. As a whole, the plottingcurve is like an ellipse.

Light having a polarization state as above is inputted to the prismsheet 12. Here, properties of the prism sheet 12 is to be considered. Asdescribed above, since the sheet material 21 as a mother material of theprism sheet 12 undergoes a two-axle drawing process, states of moleculeorientation induced in the process are different between right-half andleft-half along a width-direction.

Consequently, it is expected that polarization characteristics of theprism sheet 12 varies depending on position at which the prism sheet iscut out from the sheet material 21. In other words, transmissivity isexpected to be different depending on polarization components.

Under the above situation, the sheet material 21 was divided into ninesections 12 a through 12 i from an end toward the other end along thewidth-direction, as shown in FIG. 8, and prism sheets were prepared bycutting out from the sections, respectively. Each of the prepared prismsheet 12 a through 12 i was employed to provide a liquid crystal display1, screen brightness of which was measured. The measurement was carriedout without changing condition except exchanging of prism sheets.

It is noted that the transmission axis 16 of the polarization plate 14comprised in the liquid crystal display panel 3 employed in themeasurement is inclined 45 degrees toward the lower right in theillustration of FIGS. 13 a, 13 b with respect to an end edge 17 on theside of the fluorescent lamp 16. It is also noted that each prism sheetand a section from which the prism sheet was cut are referenced by thesame symbol.

FIG. 9 gives a table showing results. During the measurement, each cellelement in the liquid crystal cell 13 of the liquid crystal displaypanel 3 was kept in a transmission state. Brightness brought by thelight, which was transmitted through the liquid crystal display panel 3,was plotted corresponding to each of the prism sheet 12 a through 12 i.

As illustrated in FIG. 9, if any of prism sheets 12 a through 12 c isemployed, screen brightness on the liquid crystal display panel 3 was205 (cd/m²) or more. On the other hand, if any of prism sheets 12 gthrough 12 i is employed, screen brightness on the liquid crystaldisplay panel 3 was about 190 (cd/m²), falling doubtlessly. Displaybrightness obtained under employment of the prism sheet 12 g was lowerthan that obtained under employment of the prism sheet 12 a by about12%. This gives the maximum difference.

Marks ◯, Δ and X in the rightmost column in FIG. 9 express estimatedgrades, “95% or more of the maximum brightness (well-lighted)”, “notless than 90% and less than 95% of the maximum brightness(intermediately-lighted)” and “less than 90% of the maximum brightness(less-lighted)”, respectively.

These results teach that the prism sheet 12 causes difference in screenbrightness of liquid crystal display 1. Considering this situation,polarization characteristics of the prism sheets 12 a through 12 cmarked with ◯ and those of the prism sheets 12 f through 12 h markedwith X were measured for the sake of comparison.

This measurement was carried out under an arrangement as shown in FIG.7. Individual prism sheet (i.e. one of the prism sheets 12 a through 12c and 12 f through 12 h) was disposed on the emission face 11 of thelight guide plate 4 one after another and, at every disposal, an outputlight emitted in a direction of a normal 24 was examined forpolarization state. This measurement was carried out in a way like thatshown in FIG. 5. In other words, the photo-intensity measuring device 30was arranged so that the photometer 28 detected light which transmittedthrough the polarization plate 26 for measurement having thetransmission axis 27.

Intensity of light was measured at many rotation positions of thepolarization plate 26 around the direction 24. FIG. 10 is a graphicillustration to show polarization states of emissions after transmittingthrough he prism sheets 12 a to 12 c, respectively. And FIG. 11 is agraphic illustration to show polarization states of emissions aftertransmitting through he prism sheets 12 f to 12 h, respectively.

In these graphs, an angular scale extending peripherally indicatesorientational positions of the light transmission axis 27 (dual-arrow inFIG. 7) at the measurement. A radial scale indicates relative intensityof light detected by the photometer 28. The full scale is 100.

When the transmission axis 27 is orientated in accordance with thedirection 25 perpendicular to the incidence face 5, it corresponds to 0degree or 180 degrees. In the illustrated case, FIG. 7 shows a state of0 degree. The polarization plate for measurement 26 was rotatedclockwise (arrow D) from 0 degree position stepwise by 5 degrees,wherein a polarization component was measured at every rotationposition.

Referring to a graph shown in FIG. 10 (◯ mark:12 a to 12 c wereemployed), it shows plotting curves like ellipses having a longitudinalaxis 31 which is formed by polarization components corresponding toabout 335 and 155 degrees. This longitudinal axis 31 gives amaximum-intensity-direction of polarization of the emitted light.

Referring to a graph shown in FIG. 11 (X mark:12 f to 12 h wereemployed), it shows plotting curves like ellipses having a longitudinalaxis 31 which is formed by a polarization component corresponding to anangle between about 10 and 20 degrees and another polarization componentcorresponding to another angle between about 190 and 200 degrees. Thislongitudinal axis 31 gives a maximum-intensity-direction of polarizationof the emitted light.

In other words, FIG. 10 show cases giving a maximum-intensity-direction31 located at an angular position which is deviated anticlockwise byabout 25 degrees from a fiducial angular position of 0 degree-180degrees. On the other hand, FIG. 11 show cases giving amaximum-intensity-direction 31 located at an angular position which isdeviated clockwise by between about 102 and about 20 degrees from thefiducial angular position of 0 degree-180 degrees.

This teaches that the prism sheets 12 a through 12 h are, if any of thememployed, cause the liquid crystal display 1 to provide variation inscreen brightness depending on cutting position of the employed one.

Next, a prism sheet 12 manufactured by compression molding of acrylicresin, instead of the foredescribed manufacturing steps, was employed tobe subject to measurement of polarization characteristics under the samecondition. Results are shown in FIG. 12. Format of illustration is thesame as that of FIG. 10 or 11. It is noted that nothing was changed asto configuration and size of prismatic grooves.

Results shown in FIG. 12 provides a plotting curve like an ellipsehaving a longitudinal axis 31 which is formed by polarization componentscorresponding to 0 and 180 degrees. This longitudinal axis 31 gives amaximum-intensity-direction of polarization of the emitted light. Inother words, this maximum-intensity-direction 31 accords with thefiducial angular position. It is supposed that such accordance is due toabsence of molecular orientation as was induced in the case of the PETsheet 21 to which two-axle drawing applied.

It is important that the above longitudinal axis 31 in FIG. 12 has adirection which corresponds to the maximum-intensity-direction 23 ofemission from the light guide plate 4. In other words, polarizationstate at emitting from the light guide plate 4 is almost maintaineduntil after transmission through the prism sheet 12.

That is, the prism sheet 12 manufactured by compression molding ofacrylic resin does not cause the maximum-intensity-direction 31 ofpolarization of inputted light to be shifted rotationally from thefiducial angular position, if the input light is provided by emissionfrom the light guide plate 4.

The above facts attest that rotation of maximum-intensity-direction ofpolarization, which occurred under employment of the prism sheets 12 athrough 12 h, is due to none other than the PET sheet 21 to whichtwo-axle drawing applied. Viewing from another standpoint, existence ofa prismatic face is not the cause which brings themaximum-intensity-direction 31 a rotation shift.

Now, please remind that the transmission axis 16 of the polarizationplate 14 of the liquid crystal display panel 3 employed in the foresaidscreen brightness measurement (FIG. 9) is, as illustrated in FIG. 13 aor FIG. 13 b, inclined 45 degrees toward the lower right in theillustration with respect to an end edge 17 on the side of thefluorescent lamp 16.

Under this condition, the maximum-intensity-direction 31 shown in FIG.10 is inclined to a side to which the transmission axis 16 of thepolarization plate 14 is inclined. And besides, there is only a smalldifference in inclination, about 20 degrees, between the direction 31and the transmission axis 16 which has an inclination angle equal to 45degrees (FIG. 13 a).

This teaches that employment of the prism sheets 12 a through 12 ccauses the emitted light to contain an abundant-light-involvingpolarization component which meets the transmission axis 16 of thepolarization plate 14 to transmit through the polarization plate 14.

To the contrary, under the same condition as above, themaximum-intensity-direction 31 shown in FIG. 11 is inclined to anotherside (i.e. inclined to the lower left) which is opposite to foresaidside to which the transmission axis 16 of the polarization plate 14 isinclined. Crossing angle with respect to the transmission axis 16 isremarkably large, falling in a range from 55 degrees to 65 degrees (FIG.13 b).

This teaches that employment of the prism sheets 12 f through 12 hcauses the emitted light to contain a poor-light-involving polarizationcomponent which meets the transmission axis 16 of the polarization plate14 to transmit through the polarization plate 14.

From the above outcome, difference in screen brightness shown in FIG. 9can be accounted as follows.

Variation of screen brightness as shown in FIG. 9 was produced dependingon deviations of maximum-intensity-direction 31 of emission aftertransmitting through the respective prism sheets 12 a through 12 h. Inother words, the problem to which prior arts are subject is caused bythe deviations of maximum-intensity-direction 31 of emission aftertransmitting through the respective prism sheets 12 a through 12 h.

Therefore, as a conclusion, the present embodiment employs any one ofthe prism sheets 12 a to 12 c as a prism sheet to be used in combinationwith a liquid crystal display panel 3 provided with a polarization plate14 which is arranged so that the transmission direction is orientated asillustrated in FIG. 13 a or 13 b. In other words, none of the prismsheets 12 d through 12 h are not employed.

This enables the maximum-intensity-direction 31 of polarization involvedby emission from the light guide plate 4 to be rotated toward thedirection of the transmission axis 16 of the polarization plate 14,leading to an increased polarization component which is able to transmitthrough the polarization plate 14. As a result, the liquid crystaldisplay 1 is able to provide an improved screen brightness.

Another point is that products (liquid crystal displays 1) get free frombeing of uneven screen brightness by means of excluding the prism sheets12 d through 12 h intentionally.

The liquid crystal display 1 of the instant embodiment operates asfollows.

The fluorescent lamp 6 emits illumination light (primary light), whichis deflected by the light guide 4 to be converted into a flux having anenlarged cross section according to well-known processes. The flux issupplied to the liquid crystal display panel 3 via the prism sheet(light control sheet) 12.

The outputted illumination light transmits, after transmitting throughthe prism sheet 12 (any one of the prism sheets 12 a to 12 c), throughthe polarization plate 14 at a high transmissivity as explained above,being supplied to the liquid crystal display cell 13. As known well, theliquid crystal display cell 13 controls polarization state of theinputted light depending on position according to output signals of adrive circuit (not shown).

Then, output light of the liquid crystal display cell 13 transmitsthrough the polarization plate (analyser) 15 depending on state ofpolarization. From the polarization plate (analyser) 15, light isemitted with an intensity controlled depending on position to provide animage.

(2) Other Embodiments (Modifications)

The above embodiment puts no limitation on the scope of the presentinvention. For instance, the following modifications are allowed.

(a) Attention should be paid first and foremost to that the aboveembodiment chooses the prism sheets 12 a through 12 c on condition thatthe polarization plate 14 is disposed with an orientation as shown inFIG. 13 a or 13 b.

That is, if an alternatively employed liquid crystal display panel 3 isprovided with a polarization plate 14 orientated so that a transmissionaxis 16 is inclined 45 degrees toward the lower left in the illustration(FIG. 17 a) with respect to an end edge 17 on the side of thefluorescent lamp 16, the prism sheets 12 f through 12 h arealternatively employed. In other words, the prism sheets 12 a through 12g are not employed in the case.

This will be easily understood from the detailed description in theabove embodiment. Note that the alternative case, in which thetransmission axis 16 is inclined 45 degrees toward the lower left withrespect to the end edge 17 on the side of the fluorescent lamp 16,renders the transmission axis 16 inclined to an opposite side ascompared with the maximum-intensity-direction of polarization of lighttransmitted through any of the prism sheets 12 a to 12 c. Therefore, ifthe prism sheets 12 a to 12 c are employed, a reduced quantity of lightwill be able to transmit through the polarization plate 14.

To the contrary, if any of the prism sheets 12 f to 12 h are employed,the transmission axis 16 is inclined to the same side as compared withthe maximum-intensity-direction of polarization of light transmittedthrough the employed prism sheet. As a result, an increased quantity oflight will be able to transmit through the polarization plate 14.

Saying in relation to the estimation shown in FIG. 9 (marks ◯, Δ, X),the prism sheets 12 a to 12 c get marks X instead of marks ◯ while theprism sheets 12 f to 12 h get marks ◯ instead of marks X.

After all, the present invention is featured by an essential requirementthat the maximum-intensity-direction of polarization involved by lightemitted from the light guide plate 4 is rotated toward the direction oflight transmission axis of the polarization plate 14 which is disposedon the input-side of the liquid crystal display panel 3. Needless tosay, both directions accord with each other in an ideal case.

Any optional concrete structure may be employed to cause emission fromthe light guide plate 4 to involve polarization. And, any particularlimitation is not put on a concrete structure which is employed to causethe maximum-intensity-direction of polarization involved by emissionfrom the light guide plate 4 to be rotated. Examples of employablemodifications are described below.

(b) The above embodiment, prismatic grooves 19 are formed on the backface 8 of the light guide plate 4. However, this does not limit thepresent invention. For instance, prismatic grooves may be formed on theemission face 11 of the light guide plate 4. An alternatively employablelight guide plate merely contains scattering material inside withoutbeing provided with prismatic grooves.

FIG. 14 is a graphic to illustrate polarization state of emission fromthe light guide plate 4, being plotted under the measurement arrangementas shown in FIG. 5, wherein a solid-line-curve represents resultsobtained in a case where prismatic grooves 19 are formed on the backface 8 of the light guide plate 4 while a dot-line-curve representsresults obtained in another case where prismatic grooves 19 are formedon the emission face 11 of the light guide plate 4. And,double-dot-chain-line-curve represents results obtained in still anothercase where a light guide plate having no prismatic groove is employed.

As understood from FIG. 4, if prismatic grooves formed on the emissionface 11, emission involves polarization like an ellipse having alongitudinal axis bridging 0 degree-180 degrees directions, as a similarplotting is obtained when prismatic grooves 19 are formed on the backface 8. Therefore, the prism sheets 12 a to 12 c may be applied in thesame way as compared with the above embodiment, and almost the sameadvantageous effect as that of the above embodiment is obtained.

Another point to be noted is a fact a light guide plate 4 provided withno prismatic groove emits light which involves polarization like anellipse having a longitudinal axis bridging 90 degrees-270 degreesdirections, in contrast with one provided with prismatic grooves 19 onthe back face 8. Accordingly, it is reasonable in this case to choosethe prism sheets 12 f to 12 h which was estimated at X-mark in theforedescribed embodiment (FIG. 9).

This choice enables the maximum-intensity-direction 31 of emission fromthe light guide plate 4 to be rotated toward the transmission axis 16 ofthe polarization plate 14. As a result, advantageous effects areperformed in almost the same way as compared with the foredescribedembodiment.

The above three kinds of light guide plates 4 are examples of styles oflight guide plates employable in the present invention. It is noted thata style provided with prismatic grooves 19 on a back face 8 providesemission which involves a higher degree of polarization as shown in FIG.14 as compared with the other styles. Thus this style is a particularlypreferable one, since it is expected that rotation ofmaximum-intensity-direction of polarization involved by the emissioncauses quantity of light involving a polarization component capable oftransmitting through the polarization plate 14 to be increasedstrikingly.

(c) In the above embodiment, the prism sheet 12 is cut out from a sheetmember which is obtained through two-axle drawing. However, this puts nolimitation on the present invention. For instance, a prismatic face maybe formed on a PET sheet which is obtained through one-axle drawing. Itis noted that a face on which a prismatic face is to be formed is chosenunder consideration of an inclination direction of a light transmittingaxis of a polarization plate 14.

(d) Inclination direction and angle of the transmitting axis 16 of thepolarization plate 14 are not limited by the above embodiment, allowingvarious cases. Therefore, choice of the prism sheet 12 is performedunder consideration of an inclination direction of a light transmittingaxis 16 of a polarization plate 14 so that the chosen one enable themaximum-intensity-direction 31 of polarization involved by emission fromthe light guide plate 4 to be rotated toward the light transmitting axis16 of the polarization plate 14.

(e) In the above embodiment, angle of polarization-rotation achieved bythe prism sheet 12 can be brought still closer to an inclination angleof the transmission axis 16 of the polarization plate 14 by controllingfactors such as drawing conditions during two-axle drawing of the PETsheet 20. In this case, the liquid crystal display 1 will provide astill improved screen brightness.

(f) In the above embodiment, at least one light control sheet providedwith polarization-rotating ability may be interposed between the prismsheet 12 and the polarization plate 14 in order to bring themaximum-intensity-direction 31 of emission from the light guide plate 4still more closer to the direction of the transmission axis 16 of thepolarization plate 14. This enables also the liquid crystal display 1 toprovide an improved screen brightness.

(g) The above embodiment makes the sheet member 21 shoulder function ofrotating the maximum-intensity-direction of polarization involved byemission from the light guide plate 4. And further, the prismatic face22 shoulders function of correcting a propagation direction of emissionfrom the light guide plate 4. That is, both functions are shouldered bythe prism sheet 12 solely. However, instead of this way, the functionsmay be shouldered by different members respectively.

Further, for example, in cases where the propagation direction ofemission does not need to be corrected, the foresaid sheet member 21 maybe adopted solely as a light control sheet as shown in FIG. 15.

In such ways, the present invention requires a light control sheet to beat least provided with a polarization-rotatory power which rotate themaximum-intensity-direction of polarization involved by emission fromthe light guide plate 4.

(h) In the above embodiment, the employed light guide plate 4 has awedge-like cross section. However, this does not limit the presentinvention. An alternative light guide plate having an emission face 11and back face 8 extending in parallel with each other may be employed.

(i) The fluorescent lamp 6 is an example of employable primary lightsource and puts no limitation on the scope of the present invention. Forinstance, a light source provided with a linear array of light emittingdiodes may be employed alternatively.

(j) In the above embodiment, the sheet member 21 to provide a mothermaterial of the prism sheet 12 adopted as a light control sheet is madeof PET. However, this does not limit the present invention. Forinstance, other materials such as polycarbonate or acrylic resin may beemployed alternatively.

1. A liquid crystal display comprising: a liquid crystal display panel;and a surface light source device for inputting light to the liquidcrystal display panel, said liquid crystal display panel being providedwith at least a liquid crystal cell and a polarization plate arranged atan input side of the liquid crystal cell, said surface light sourcedevice being provided with at least a light guide plate emitting lighthaving a polarization state, a primary light source to supplyillumination light to said light guide plate and a light control sheetinterposed between said light guide plate and said polarization plate,wherein said light control sheet is made of a particular portion of aresin material produced through a resin material drawing process and isprovided with an ability acquired through the resin material drawingprocess and a cutting-out process for choosing the particular portionsuch that a maximum-intensity-direction of polarization of the lightemitted from said light guide plate is rotated around a travelingdirection of the light toward a direction of a light transmission axisof said polarization plate by transmitting through the light controlsheet.
 2. A liquid crystal display as recited in claim 1, wherein saidlight control sheet is a prism sheet disposed next to said polarizationplate.
 3. A surface light source device for inputting light to a liquidcrystal display panel provided with at least a liquid crystal cell and apolarization plate arranged at an input side of the liquid crystal cell,comprising: at least a light guide plate emitting light having apolarization state; a primary light source to supply illumination lightto the light guide plate; and a light control sheet interposed betweensaid light guide plate and said polarization plate, wherein said lightcontrol sheet is made of a particular portion of a resin materialproduced through a resin material drawing process and is provided withan ability acquired through the resin material drawing process and acutting-out process for choosing the particular portion such that amaximum-intensity-direction of polarization of the light emitted fromsaid light guide plate is rotated around a traveling direction of thelight toward a direction of a light transmission axis of saidpolarization plate by transmitting through the light control sheet.
 4. Asurface light source device as recited in claim 3, wherein said lightcontrol sheet is a prism sheet disposed next to said polarization plate.5. A light control sheet arranged for inputting light to a liquidcrystal display panel provided with a least a liquid crystal cell and apolarization plate which is arranged at an input side of the liquidcrystal cell, the light control sheet being applied to a surface lightsource device provided with at least a light guide plate emitting lighthaving a polarization state and a primary light source to supplyillumination light to the light guide plate, wherein said light controlsheet is made of a particular portion of a resin material producedthrough a resin material drawing process and is provided with an abilitythat acquired through the resin material drawing process and acutting-out process for choosing the particular portion amaximum-intensity-direction of polarization of the light emitted fromsaid light guide plate is rotated around a traveling direction of thelight toward a direction of a light transmission axis of saidpolarization plate by transmitting through the light control sheet.
 6. Alight control sheet as recited in claim 5, wherein said light controlsheet is a prism sheet disposed next to said polarization plate.